Method for producing compound semiconductor light-emitting device

ABSTRACT

It is intended to provide a production method that enables at least one of improvement in transparency, reduction in sheet resistance, homogenization in planar distribution of sheet resistance, and reduction in contact resistance related to a contact layer regarding a transparent conductive oxide film included in a compound semiconductor light-emitting device. A method for producing a compound semiconductor light-emitting device includes depositing on a substrate a compound semiconductor stacked-layer body including a light-emitting layer, depositing a transparent conductive oxide film on the compound semiconductor stacked-layer body, and annealing the transparent conductive oxide film and thereafter cooling the same in a vacuum atmosphere.

This nonprovisional application is based on Japanese Patent ApplicationNo. 2010-220779 filed on Sep. 30, 2010 with the Japan Patent Office, theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a method for producing a compoundsemiconductor light-emitting device and particularly to a productionmethod that can improve at least one of transparency, sheet resistance,planar distribution of sheet resistance, and contact resistanceregarding a transparent conductive oxide film included in a compoundsemiconductor light-emitting device.

2. Description of the Background Art

Compound semiconductor light-emitting devices that can emit the threeprimary color lights of red, green and blue are indispensable in orderto utilize the light-emitting devices for various illumination uses.Regarding light-emitting diodes (LEDs), it has not been possible untilrecent years to utilize LEDs for various illumination uses because theblue LED among LEDs of the three primary colors has not beenwell-completed and not been available.

However, after the blue LED formed with nitride semiconductor has beendeveloped in the 1990s, illumination products including LEDs areutilized not only for traffic signals but also for backlights in liquidcrystal monitors, backlights in liquid crystal televisions and furthervarious illumination uses at home.

Recently, liquid crystal televisions equipped with LED backlights beginto become widely used in a rapid pace in association with their pricedecline. In addition, illumination devices using LEDs have merits ofenabling lower power consumption, smaller space occupied by them, andfree of mercury preferably to the environment, as compared with theconventional illumination devices. After the summer of 2009,illumination devices using LEDs have been put on the market at much lessprices as compared with those before and thus become popular in a veryrapid pace.

In the meantime, light emitted from an illumination device, a backlightof a liquid crystal television, or the like should necessarily be whitelight. In general, white light obtainable using an LED can be realizedby a combination of a blue LED and a yellow YAG(yttrium-aluminum-garnet) phosphor or a combination of a blue LED, agreen phosphor and a red phosphor. In other words, a blue LED is neededin the case of obtaining white light utilizing an LED. For this reason,it is desired to provide a method that can produce bright blue LEDs inlarge amounts at low prices.

In general, III-V compound semiconductors containing nitrogen as aV-group element, such as gallium nitride (GaN), aluminum nitride (AlN),indium nitride (InN) and mixed crystals thereof are used forlight-emitting layers included in LEDs and laser diodes (LDs) that canemit lights of shorter wavelengths such as blue and bluish green lights.

FIG. 6 shows a blue LED of a well-known double hetero-junction type in aschematic cross-sectional view. In production of this blue LED, anSi-doped n-type GaN lower clad layer 102, an InGaN light-emitting layer103, a Mg-doped p-type AlGaN upper clad layer 104, and a contact layer105 are deposited in this order on a sapphire substrate 101. Depositedon contact layer 105 is a transparent conductive oxide film 108 on apartial area of which a p-side electrode 106 is provided. On the otherhand, lower clad layer 102 is partly exposed by etching and then ann-side electrode 107 is provided on the exposed area.

When electric current is injected through p-side electrode 106 in theLED of FIG. 6, the current is dispersed in the planar direction ofconductive oxide film 108. Then, the dispersed current is injectedthrough contact layer 105 and upper clad layer 104 into a broad area oflight-emitting layer 103 thereby causing light emission from the broadarea of light-emitting layer 103.

Light emitted upward from light-emitting layer 103 is transmittedthrough upper clad layer 104, contact layer 105 and transparentconductive oxide film 108 and then taken out to the outside. By using ahighly transparent material such as ITO (indium tin oxide) fortransparent conductive oxide film 108, it becomes possible to reducelight loss when light emitted from light-emitting layer 103 istransmitted through transparent conductive oxide film 108. Further,since conductive oxide film 108 such as of ITO has lower resistance ascompared with contact layer 105, diffusion of the injected current isenhanced to spread widely so as to increase light-emitting area oflight-emitting layer 103 thereby improving the light-emitting efficiencyof the device.

On the other hand, while conductive oxide film 108 has lower resistanceas compared with contact layer 105, it exhibits a relatively high sheetresistance value in a range of 20Ω/□ to 60Ω/□. Further, the conductiveoxide film tends to include relatively higher and lower sheet resistanceareas in a mixed state depending on its partial areas. Therefore, thecompound semiconductor light-emitting device including the transparentconductive oxide film has problems in which an operation voltage Vfthereof tends to become higher and light emission therefrom tends tobecome non-uniform depending on partial areas of the light-emittinglayer.

In order to improve these problems, it is ideal to reduce the sheetresistance value of transparent conductive oxide film 108 to 20Ω/□ orless and preferably to 10Ω/□ or less. For the purpose of reducing thesheet resistance of the conductive oxide film, it is conceivablypossible to improve the crystallinity of the conductive oxide film byannealing so as to reduce the sheet resistance of the conductive oxidefilm. However, there is a problem that a bonding state at an interfacebetween conductive oxide film 108 and contact layer 105 is changed bythe annealing to impair a stable bonding state such as of Ga—O, N—O,compounds of H, etc. at the interface, leading to higher contactresistance.

For the purpose of reducing the sheet resistance of the conductive oxidefilm, it is also conceivably possible to increase oxygen defect densityin the crystal thereby increasing charge carrier density. However, thework function of the conductive oxide film is decreased as the carrierdensity is increased, whereby the electron potential is increased on theconductive oxide film side at the interface between conductive oxidefilm 108 and contact layer 105. For this reason, holes are poorlyinjected from the conductive oxide film to the contact layer, therebycausing a problem of increasing the contact resistance betweenconductive oxide film 108 and contact layer 105.

Under the circumstances, in the light-emitting device disclosed inJapanese Patent Laying-Open No. 2007-287786, it is attempted to reducethe sheet resistance of the transparent conductive oxide film withmaintaining low contact resistance between the conductive oxide film andthe contact layer by carrying out two-stage annealing including firstannealing and second annealing. In the first annealing of JapanesePatent Laying-Open No. 2007-287786, the annealing is carried out at atemperature in a range of 250° C. to 600° C. in an atmosphere containingoxygen so as to reduce the contact resistance between the transparentconductive oxide film and the contact layer. Subsequently, in the secondannealing, the annealing is carried out at a temperature in a range of200° C. to 500° C. in an atmosphere free of oxygen (e.g., a N₂ gasatmosphere) so as to reduce the sheet resistance of the transparentconductive oxide film.

It is conventionally usual that an oxide film such as of ITO or IZO(indium zinc oxide) used for the transparent conductive oxide film isannealed in a N₂ gas atmosphere at the same pressure as the atmosphericpressure and then is cooled in the same N₂ gas atmosphere and then takenout from the furnace or cooled in a flow of an inert gas such as N₂ evenin the case that the oxide film is annealed in a vacuum atmosphere. Withthis method, however, it is not possible to sufficiently reduce thesheet resistance of the transparent conductive oxide film, and there arecaused problems of increase in operation voltage of the light-emittingdevice, non-uniform light emission in the surface of the light-emittinglayer, and so forth.

SUMMARY OF THE INVENTION

In view of the problems as above, an object of the present invention isto provide a method for producing a compound semiconductorlight-emitting device, which enables at least one of improvement intransparency, reduction in sheet resistance, homogenization in planardistribution of sheet resistance, and reduction in contact resistancerelated to a contact layer, regarding a transparent conductive oxidefilm included in the compound semiconductor light-emitting device.

As a result of various investigations, the present inventors have foundthat the oxide film such as of ITO or IZO used for the transparentconductive oxide film shows better homogeneity in planar distribution ofsheet resistance in the case of having been cooled in a vacuumatmosphere rather than in a nitrogen atmosphere after annealing, thoughboth the atmospheres are the same in terms of atmospheres free ofoxygen.

A method for producing a compound semiconductor light-emitting deviceaccording to an aspect of the present invention includes depositing on asubstrate a compound semiconductor stacked-layer body including alight-emitting layer, depositing a transparent conductive oxide film onthe compound semiconductor stacked-layer body, and annealing thetransparent conductive oxide film and then cooling the same in a vacuumatmosphere.

The annealing is preferably conducted in a gas atmosphere free of oxygenand may be carried out in a nitrogen gas atmosphere, in an argon gasatmosphere, or in a mixed gas atmosphere of nitrogen gas and argon gas.The annealing may also be carried out in a vacuum atmosphere.

A method for producing a compound semiconductor light-emitting deviceaccording to another aspect of the present invention includes depositingon a substrate a compound semiconductor stacked-layer body including alight-emitting layer, depositing a transparent conductive oxide film onthe compound semiconductor stacked-layer body, and subjecting thetransparent conductive oxide film to first annealing in an atmospherecontaining oxygen and then to second annealing in an atmosphere free ofoxygen and thereafter cooling the same in a vacuum atmosphere.

The second annealing is preferably conducted in a gas atmosphere free ofoxygen and may be carried out in a nitrogen gas atmosphere, in an argongas atmosphere, or in a mixed gas atmosphere of nitrogen gas and argongas. The second annealing may also be carried out in a vacuumatmosphere.

In the method for producing a compound semiconductor light-emittingdevice according to either of the aspects of the present invention,pressure in the vacuum atmosphere is preferably 10 Pa or less. Thecooling in the vacuum atmosphere is preferably continued to atemperature of 200° C. or less. The transparent conductive oxide film ispreferably formed of an oxide including indium and may be formed of ITOor IZO. The transparent conductive oxide film preferably has a thicknessin a range of 100 nm to 400 nm.

With the production method as described above, it becomes possible toprovide a compound semiconductor light-emitting device having variousimproved characteristics.

More specifically, according to the present invention, it becomespossible to realize at least one of improvement in transparency,reduction in sheet resistance, homogenization in planar distribution ofsheet resistance, and reduction in contact resistance related to acontact layer, regarding a transparent conductive oxide film included ina compound semiconductor light-emitting device and then it becomespossible to obtain a compound semiconductor light-emitting device thathas a reduced operation voltage and can provide uniform light emission.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing an example of a compoundsemiconductor light-emitting device that can be made by a method of thepresent invention.

FIG. 2 is a schematic cross-sectional view taken along a line X-X shownin FIG. 1.

FIG. 3 is a schematic graph showing an example of a temperature profileduring annealing of a transparent conductive oxide film and duringcooling thereafter.

FIG. 4 is a histogram showing a comparison between averaged sheetresistance values of transparent conductive oxide films in the cases ofhaving been cooled in a gas atmosphere and in a vacuum atmosphere afterannealing.

FIG. 5 is a histogram showing a comparison between degrees of planaruniformity of sheet resistance in transparent conductive oxide films inthe cases of having been cooled in a gas atmosphere and in a vacuumatmosphere after annealing.

FIG. 6 is a schematic cross-sectional view showing an example of a blueLED of a well-known double hetero junction type.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Compound Semiconductor Light-Emitting Device>

FIG. 1 shows in a schematic perspective view an example of a compoundsemiconductor light-emitting device that can be produced by a method ofthe present invention, and FIG. 2 is a cross-sectional view taken alonga line X-X shown in FIG. 1. In the drawings of the present application,the dimensional relations such as length, width and thickness arearbitrarily changed for clarity of the drawings and thus do not show theactual dimensional relations.

In the compound semiconductor light-emitting device, a lower clad layer2, a light-emitting layer 3, an upper clad layer 4, and a contact layer5 are deposited in this order on a substrate 1. Formed on contact layer5 is a transparent conductive oxide film 8, on a partial area of which afirst electrode 6 is provided. On the other hand, a part of lower cladlayer 2 is exposed by etching and then a second electrode 7 is providedon the exposed area.

Here, a double hetero junction is formed with lower clad layer 2,light-emitting layer 3 and upper clad layer 4. For light-emitting layer3, it is possible to select a compound semiconductor of being undoped,an n-type, a p-type, or containing impurities for both the n-type andp-type, as desired. A p-n junction is formed between lower clad layer 2and upper clad layer 4 with light-emitting layer 3 therebetween.

<Lower Clad Layer>

In the light of technical meaning of the double hetero junctionstructure, each of the clad layers has a greater bandgap as comparedwith the light-emitting layer and has a function of holding electronsand holes within the light-emitting layer by a potential barrier due tothe gap difference. However, lower clad layer 2 in FIG. 1 and FIG. 2 mayalso include a buffer layer between substrate 1 and light-emitting layer3 as well as a contact layer for good ohmic contact with n-sideelectrode 7. Such lower clad layer 2 can be formed as a multiple layerincluding not only a nitride semiconductor layer doped with n-typeimpurities but also undoped nitride semiconductor layer, for example.More specifically, lower clad layer 2 can include a low temperaturebuffer layer, an AlN buffer layer, an undoped layer, a doped layer of ann-type, an n-type contact layer, and so forth, for example.

As described above, while lower clad layer 2 may be formed as a singlelayer or a multiple layer functioning as a clad layer, GaN, AlGaN,InAlGaN, or InGaN can be used in the case of the single layer and maycontain Si or may be undoped. Further, in the case of lower clad layer 2being formed as a multiple layer, it may have a stacked-layer structureof InGaN/GaN, InGaN/AlGaN, AlGaN/GaN, InGaN/InGaN, or the like and mayhave a periodic multilayer structure including a plurality of layersstacked repeatedly. Further, such a multiple layer structure may beformed as a super lattice structure.

<Light-Emitting Layer>

Light-emitting layer 3 is preferably formed by alternately stacking GaNbarrier layers and well layers of nitride semiconductor containing In.While preferable thickness of the well layer depends on wavelength oflight to be emitted, it is preferably in a range of 2 to 20 nm. Suchlight-emitting layer 3 is not restricted to have a quantum wellstructure but may have any of a single well structure, a multiple wellstructure, a multiple quantum well structure, and the like. In the casethat light-emitting layer 3 includes a plurality of well layers, it isonly necessary that at least one well layer accomplishes thelight-emitting function. Such a well layer is preferably formed withIn_(p)Ga_(1-p)N (0<p<1), for example.

<Upper Clad Layer>

As described above, in the light of technical meaning of the doublehetero junction structure, each of the clad layers has a greater bandgapas compared with the light-emitting layer and has a function of holdingelectrons and holes within the light-emitting layer by a potentialbarrier due to the gap difference. However, upper clad layer 4 in FIG. 1and FIG. 2 may also include an evaporation-prevention layer, acarrier-blocking layer and a p-type layer serving as a current diffusionlayer. In other words, upper clad layer 4 can be a single layer or amultiple layer. Any of GaN, AlGaN, InAlGaN, and InGaN that are undopedor doped with p-type impurities can be used for the single layer. In thecase of upper clad layer 4 being formed as a multiple layer, it may havea stacked-layer structure of InGaN/GaN, InGaN/AlGaN, AlGaN/GaN,InGaN/InGaN, or the like and may have a periodic multilayer structureincluding a plurality of layers stacked repeatedly. Further, such amultilayer structure may be formed as a super lattice structure.

Such an upper clad layer preferably has a thickness of 500 nm or less.The reason for this is that if upper clad layer having a thickness morethan 500 nm is deposited by vapor phase deposition, light-emitting layer3 is subjected to heat for a long time and then non-light-emittingpartial areas on light-emitting layer 3 are increased due to thermaldegradation. In the meantime, it is preferable to provide anevaporation-prevention layer in contact with light-emitting layer 3 inorder to prevent In contained in the light-emitting layer fromevaporating. The evaporation-prevention layer can be included in upperclad layer 4, as described above.

<Contact Layer>

Contact layer 5 is provided so as to reduce the contact resistance withtransparent conductive oxide film 8. Such contact layer 5 is preferablyof nitride semiconductor doped with a higher concentration of p-typeimpurities as compared with upper clad layer 4. It should be noted thattransparent conductive oxide film 8 may be formed on upper clad layer 4without providing contact layer 5. In this case, it is preferable toincrease the concentration of p-type impurities in the vicinity of theupper surface of upper clad layer 4.

<First Electrode and Second Electrode>

First electrode 6 and second electrode 7 serve as bases for wire-bondingfor electrically connecting with an outer circuit. First electrode 6 andsecond electrode 7 can be formed in a known aspect by utilizingmaterials of Ti, Al, Au, etc. Each of first electrode 6 and secondelectrode 7 is not restricted to be a single layer and can be formedwith a multilayer structure. In the case of each of first electrode 6and second electrode 7 having a multilayer structure, the uppermostlayer is preferably formed as an Au layer having a thickness of about500 nm. This makes it possible to secure stability of wire-bonding withan outer circuit when the compound semiconductor light-emitting deviceis mounted on a package.

In the meantime, part of light emission from light-emitting layer 3 isemitted toward the side of upper clad layer 4. Therefore, firstelectrode 6 is an electrode positioned in a direction in which lightemitted from light-emitting layer 3 is taken out of the side of upperclad layer 4. On the other hand, second electrode 7 in FIG. 1 and FIG. 2is shown on an exemplary position in the case of substrate 1 beingformed of an insulative material. Specifically, second electrode 7 isprovided on an exposed partial area of lower clad layer 2 in the case ofsubstrate 1 being formed of an insulative material. In the case ofsubstrate 1 being formed of a conductive material, however, secondelectrode 7 can be formed on the bottom surface of substrate 1.

<Transparent Conductive Oxide Film>

Transparent conductive oxide film 8 is provided so as to transmit lightemitted from light-emitting layer 3 and so as to make a contact withcontact layer 5 whereby diffusing current to the entire surface thereofand thus increasing the light-emitting area of light-emitting layer 3below. Therefore, it is preferable to use a material having lowerresistance for transparent conductive oxide film 8 as compared withcontact layer 5. This makes it possible that current injected from firstelectrode 6 is diffused in the planar direction of transparentconductive oxide film 8. Such conductive oxide film 8 can preferably beformed by utilizing materials of ITO, IZO, etc. and it is particularlypreferable to use ITO. The reason for this is that ITO is particularlyexcellent from the viewpoint of the transparency and the contactresistance.

Transparent conductive oxide film 8 preferably has a thickness in arange of 100 nm to 400 nm. The reason for this is that if transparentconductive oxide film 8 has a thickness less than 100 nm, the high sheetresistance thereof causes increase of operation voltage of thelight-emitting device and if it has a thickness greater than 400 nm, thetransparency thereof is decreased and then light extraction efficiencyof the light-emitting device is decreased.

<Method for Producing Compound Semiconductor Light-Emitting Device>

In a method for producing a compound semiconductor light-emitting deviceaccording an embodiment of the present invention, semiconductor layers 2to 5 of compound semiconductor such as III-group nitride semiconductorare deposited in this order on substrate 1. Transparent conductive oxidefilm 8 is then deposited on contact layer 5 and this conductive oxidefilm 8 is subjected to first annealing in an atmosphere containingoxygen and subsequently to second annealing in an atmosphere free ofoxygen and then subjected to cooling in vacuum.

The oxide such as ITO or IZO used for the transparent conductive oxidefilm is conventionally subjected to annealing in a N₂ atmosphere at thesame pressure as the atmospheric pressure and directly cooled in thesame N₂ atmosphere and then taken out from the furnace. However, thisconventional method does not make it possible to sufficiently reduce thesheet resistance of the conductive oxide film and causes increase ofoperation voltage of the light-emitting device and non-uniform lightemission in the light-emitting layer.

The method of the present invention can improve those conventionalproblems and bring about at least one of improvement in transparency,reduction in contact resistance related to contact layer 5, andimprovement in planar uniformity of sheet resistance regardingtransparent conductive oxide film 8 by cooling the oxide film in vacuumafter annealing. Each step of the method for producing the compoundsemiconductor light-emitting device will be explained in more detail inthe following.

<Deposition of Compound Semiconductor Layers>

The temperature of substrate 1 is first adjusted to 1050° C. for examplein an MOCVD (metal organic chemical vapor deposition) apparatus and thenlower clad layer 2 is crystal-grown on substrate 1 by introducing aIII-group element source gas with a carrier gas containing nitrogen andhydrogen, a doping gas containing Si, and ammonia gas into theapparatus.

Here, as the III-group element source gas to be introduced into theMOCVD apparatus for growing lower clad layer 2, it is possible to useTMG ((CH₃)₃Ga:trimethylgallium), TEG ((CH₂)₅Ga:triethylgallium), TMA((CH₃)₃Al:trimethylaluminum), TEA ((CH₂)₅Al:triethylaluminum), TMI((CH₃)₃In:trimethylindium), or TEI ((CH₂)₅In:triethylindium), forexample. Further, it is possible to use SiH₄ (silane) for example as thedoping gas.

Subsequently, light-emitting layer 3 is formed by alternately depositingwell layers containing In and barrier layers on lower clad layer 2.Further, upper clad layer 4 is formed on light-emitting layer 3. At thistime, the substrate temperature is adjusted to a temperature suitablefor crystal-growing upper clad layer 4. Then, upper clad layer 4 iscrystal-grown on light-emitting layer 3 by introducing a carrier gascontaining nitrogen and hydrogen, a III-group element source gas, adoping gas containing Mg, and ammonia gas into the MOCVD apparatus.Subsequently, contact layer 5 is formed on upper clad layer 4.

Specifically, in the case that upper clad layer 4 is to be formed withGaN or AlGaN, the substrate temperature suitable for crystal-growingupper clad layer 4 is preferably in a range of 950° C. to 1300° C. andmore preferably in a range of 1000° C. to 1150° C. By crystal-growingupper clad layer 4 at such a substrate temperature, it is possible toimprove crystallinity of upper clad layer 4.

As the doping gas containing Mg, it is possible to use Cp₂Mg(biscyclopentadienyl magnesium) or (EtCp)₂Mg (ethyl biscyclopentadienylmagnesium), for example. Incidentally, as compared with Cp₂Mg being in asolid state under conditions of a room temperature and an atmosphericpressure, (EtCp)₂Mg being in a liquid state under the same conditionshas better responsiveness to change in flow rate into the MOCVDapparatus and thus the vapor pressure thereof can be readily maintainedto be constant.

As the III-group element source gas and ammonia gas used for formingupper clad layer 4, it is possible to utilize the some kinds of gases asthose used in the case of forming lower clad layer 2 and light-emittinglayer 3. By the processes as above, it is possible to form a compoundsemiconductor stacked-layer body including lower clad layer 2,light-emitting layer 3, upper clad layer 4, and contact layer 5.

<Deposition of Transparent Conductive Oxide Film>

Transparent conductive oxide film 8 is deposited on contact layer 5 byusing an electron beam evaporation method or a sputtering evaporationmethod. Incidentally, it is possible to deposit transparent conductiveoxide film 8 directly on upper clad layer 4 without contact layer 5therebetween. In the case of depositing transparent conductive oxidefilm 8 by a sputtering evaporation method, transparent conductive oxidefilm 8 is formed by introducing a target and a sputtering gas into asputtering chamber in which sputtering power is then applied.

<First Annealing>

Transparent conductive oxide film 8 deposited as described above issubjected to first annealing in an atmosphere containing oxygen. Bycarrying out the annealing in such an atmosphere containing oxygen, itis possible to crystallize the material constituting transparentconductive oxide film 8, improve the transparency, and reduce thecontact resistance related to contact layer 5. Namely, the annealing inan atmosphere containing oxygen can improve the cystallinity oftransparent conductive layer 8 and reduce the sheet resistance ofconductive oxide film 8.

In the meantime, it is preferable to carry out the first annealing at atemperature in a range of 450° C. to 700° C. This makes it possible toenhance the effect of reducing the sheet resistance of conductive oxidefilm 8. Further, the first annealing is conducted preferably for a timein a range of 3 minutes to 30 minutes and more preferably for a time of20 minutes or less.

<Second Annealing>

After the first annealing, transparent conductive oxide film 8 issubjected to second annealing at an atmosphere free of oxygen. The mostimportant feature of the present invention resides in that cooling iscarried out in vacuum after the second annealing.

The annealing in an atmosphere free of oxygen can reduce the sheetresistance of the transparent conductive oxide film without degradingthe transparency thereof. Further, it is possible by cooling in vacuumto homogenize the planar distribution of sheet resistance in thetransparent conductive oxide film.

The second annealing is conducted preferably in a vacuum atmosphere, anitrogen atmosphere, an argon atmosphere, or a mixed atmosphere ofnitrogen and argon, and most preferably in the vacuum atmosphere. Thevacuum atmosphere during the cooling means an atmosphere of a pressuresignificantly reduced as compared to an atmospheric pressure and itspressure is preferably 10 Pa or less. Further, it is preferable tocontinue the vacuum cooling to a temperature of 200° C. or less.

A graph of FIG. 3 exemplarily and schematically shows a temperatureprofile during the second annealing and cooling thereafter. In thisgraph, therefore, the vertical axis represents the temperature in anannealing furnace and the horizontal axis represents the time. Further,the solid line in the graph represents the vacuum state in the furnaceand the broken line represents the state of an atmospheric pressure inthe furnace.

With the second annealing conducted in a vacuum atmosphere, it ispossible to form oxygen defects in the crystal of the transparentconductive oxide film so as to increase the carrier density and thenreduce the sheet resistance of the conductive oxide film. Furthermore,by cooling in the vacuum atmosphere of the same pressure as that duringthe annealing, it is possible to improve the uniformity of planardistribution of sheet resistance in the conductive oxide film.

A histogram of FIG. 4 shows a comparison between averaged sheetresistance values of the conductive oxide films in the cases of havingbeen cooled in gas and in vacuum after the second annealing. In thisgraph, the vertical axis represents the relative sheet resistance (Ω/□),the left side bar shows the sheet resistance in the case of cooling in agas, and the right side bar shows the sheet resistance in the case ofcooling in vacuum. From the result of FIG. 4, it is understood that thedifference between the cooling in gas and the cooling in vacuum does notexert significant influence on the averaged sheet resistance value ofthe conductive oxide film.

In the meantime, it is preferable to conduct the second annealing at atemperature not higher than the temperature of the first annealing.Further, the second annealing is preferably conducted for a time in arange of one minute to 30 minutes. With the second annealing, it ispossible to obtain a sheet resistance value of 10Ω/□ or less intransparent conductive oxide film 8 without deterioration of thetransparency and increase of the contact resistance related to contactlayer 5.

A histogram of FIG. 5 shows a comparison between degrees of planaruniformity of sheet resistance in the transparent conductive oxide filmsin the cases of having been cooled in gas and in vacuum after the secondannealing. As shown in FIG. 5, while variation of the sheet resistancein the plane of the conductive oxide film having been cooled in gas isabout 5%, variation of the sheet resistance in the plane of theconductive oxide film having been cooled in vacuum is about a half andspecifically about 2% whereby better planar uniformity of the sheetresistance is obtained.

The reason for this can be considered as follows. At the time just afterbeginning of the cooling after the annealing, the conductive oxide filmstill remains at a high temperature enough to generate oxygen defects.In the case of cooling in gas in such a state, it is considered that atemperature distribution is caused by the cooling gas. Therefore, it isthen considered that oxygen defects are further generated in someportions in the conductive oxide film and the other portions are cooledwithout further generation of oxygen defects, whereby degrading theplanar uniformity of the sheet resistance. In the case of cooling invacuum after the annealing, on the other hand, it is considered thatsince the temperature distribution due to the cooling gas does nothappen, the entire conductive oxide film is uniformly cooled and thusvariation in the planar distribution of the sheet resistance becomessmall.

Incidentally, the averaged sheet resistance value in FIG. 4 was obtainedas an arithmetic mean value calculated from values measured by afour-terminal method at 73 points on the conductive oxide film. Further,the planar uniformity degree of the sheet resistance in FIG. 5 wascalculated by dividing the difference between the maximum and minimumsheet resistance values at the 73 points by the double of the averagedsheet resistance value.

<Formation of First and Second Electrodes>

First electrode 6 and second electrode 7 described above can be formedby utilizing known photolithography, known electron beam evaporation, aknown lift-off process, and so forth.

Although the embodiments of the present invention have been describedand illustrated in detail, it is also intended to select and combine thevarious technical matters disclosed in the embodiments. Further, itshould be understood that the embodiments are by way of illustrationsand examples only and are not to be taken by way of limitations, thescope of the present invention being interpreted by the terms of theappended claims, and it is intended that the present invention includesall the variations within the meaning and scope equivalent to the scopeof the claims.

The production method of the present invention can improve at least oneof the transparency, sheet resistance, planar distribution of sheetresistance, and contact resistance of a transparent conductive oxidefilm included in a compound semiconductor light-emitting device, andthen such an improved compound semiconductor light-emitting device canpreferably be utilized in LED illumination, backlight of a liquid TV,and so forth.

What is claimed is:
 1. A method for producing a compound semiconductorlight-emitting device, comprising: depositing on a substrate a compoundsemiconductor stacked-layer body including a light-emitting layer;depositing a transparent conductive oxide film on the compoundsemiconductor stacked-layer body; subjecting the transparent conductiveoxide film to first annealing in a gaseous atmosphere containing oxygen;subjecting, after the first annealing, the transparent conductive oxidefilm to second annealing in a vacuum atmosphere or in a gaseousatmosphere free of oxygen; and cooling, after the second annealing, thetransparent conductive oxide film in a vacuum atmosphere.
 2. The methodaccording to claim 1 wherein pressure in the vacuum atmosphere duringthe annealing is 10 Pa or less.
 3. The method according to claim 1wherein the cooling in the vacuum atmosphere is continued to atemperature of 200° C. or less.
 4. The method according to claim 1wherein the transparent conductive oxide film is formed of an oxideincluding indium.
 5. The method according to claim 4 wherein thetransparent conductive oxide film is formed of ITO or IZO.
 6. The methodaccording to claim 1 wherein the transparent conductive oxide film has athickness in a range of 100 nm to 400 nm.
 7. The method according toclaim 1 wherein the second annealing is carried out only in the gaseousatmosphere free of oxygen.
 8. The method according to claim 7 whereinthe second annealing is carried out in a nitrogen gas atmosphere, in anargon gas atmosphere, or in a mixed gas atmosphere of nitrogen gas andargon gas.
 9. The method according to claim 1 wherein the secondannealing is carried out only in the vacuum atmosphere.
 10. A compoundsemiconductor light-emitting device produced by the following processsteps: depositing on a substrate a compound semiconductor stacked-layerbody including a light-emitting layer; depositing a transparentconductive oxide film on the compound semiconductor stacked-layer body;subjecting the transparent conductive oxide film to first annealing in agaseous atmosphere containing oxygen; subjecting, after the firstannealing, the transparent conductive oxide film to second annealing ina vacuum or in a gaseous atmosphere free of oxygen; and cooling, afterthe second annealing, the transparent conductive in a vacuum atmosphere,wherein a planar distribution of a sheet resistance of the transparentconductive oxide film is more homogenized than a planar distribution ofa sheet resistance of a transparent conductive oxide film that is notcooled in the vacuum atmosphere.